The present invention relates to the field of electrochemistry, and more specifically to the characterizing the integrity of diffusion barriers, particularly diffusion barriers in electronic interconnects.
Current electronics typically incorporate multiple layers of complex interconnect arrays that carry signals to and from its various components. State of the art electronics are comprised of layers of thin film processed metal interconnects (often copper) acting as conductors as well as dielectric materials that are electrically isolated from the conductors. In order to prevent diffusion and chemical reactions between the conducting material and the dielectric material, a thin layers of metal (typically refractory metal or similar material) is typically placed between the conducting and dielectric materials acting a diffusion barrier.
As smaller and higher performance interconnects become increasingly important, the reliability of the diffusion barrier has become critical. Barrier reliability is especially crucial for future technologies as the thickness of these diffusion barriers approach nanoscale dimensions and the barriers are integrated with dielectric materials (e.g., porous low-dielectric [low-κ] materials) that will impose more aggressive mechanical and chemical loads. With such very thin diffusion barriers, even small defects are capable of compromising the integrity of the barrier and instigating various types of functional and physical failures of the copper or dielectric material within the interconnect structure. In addition, relatively thick barriers (e.g., a 25 nm-thick Ta diffusion barrier that was sputter-deposited on MSSQ based porous low-κ dielectric) are capable of failing or, at a minimum, allowing the out-migration and flooding of copper to the dielectric area (e.g., low-κ area) nearby, leaving extensive voids behind.
Such defects and/or failures of diffusion barriers are instigated by at least two parameters, including ambient and defects in the barrier. Out-migration of Cu, for example, is generally found to be driven by an oxidation potential provided by ambient or a stress gradient. Failure in barrier integrity is generally triggered by one or more defects in the barrier. The defect may be present when the barrier was deposited or may develop during subsequent processing/fabrication of the interconnect. To protect from such failure, strategies for eliminating the driving forces for Cu out-diffusion such as ambient infiltration and ways to improve barrier quality are in order. Unfortunately, these are technically challenging, because near perfect barrier coverage must be achieved where there is a less thick structure and where physical support for the barrier layer is lacking. More challenging too is how to characterize harmful and/or fatal defects in these barriers.
Several methods have been used to characterize the reliability and integrity of a barrier layer. These include a direct observation of the barrier microstructure using microscopy, measurement of dielectric break-down, biased thermal stressing (BTS), stress migration (SM) testing, and electromigration (EM) testing. Unfortunately, the above methods were developed for interconnects with dense dielectric and thick barrier layers. As such, they are ineffective for detecting harmful and/or fatal defects in current interconnects, especially those comprising sub-microscopic (e.g., nanoscale) diffusion barriers. Further, the above methods are time consuming and not specifically designed to examine barrier quality which increases their potential for false diagnosis.
The danger of false diagnosis increases when a metallic diffusion barrier is coupled with a pore-seal layer. The pore-seal layer is typically placed to increase structural and/or chemical stability of the barrier. A pore seal is typically comprised of a thin layer of dense dielectric material deposited prior to deposition of the metallic barrier layer. Ideally, the pore-seal prevents ingress of processing gases and/or liquids into the dielectric layer (e.g., a porous low-κ dielectric) and also provides mechanical support for the barrier and for the interconnect. Failure of the pore-seal—either completely or partially—exposes the barrier portion and thin film processed metal portion of the interconnect to the same types of failure as described above. Further, with a pore-seal that is defective, the defect and/or failure is typically local and less extensive and thus, more difficult to detect. Current methods are unable to detect defects and/or failure of a pore seal without extensive and time-consuming examination generally involving several steps and/or equipment.
Therefore, there remains a need to develop a method for evaluating diffusion barrier integrity and for detecting defects in a diffusion barrier and a pore-seal.